1. Field of the Invention
The present invention relates to a color separation filter for use in a color copier and, more particularly, a filter to improve sideband characteristics.
2. Description of the Prior Art
In an electrostatic copying system, a document placed on a transparent plate is illuminated by an appropriate illuminating device, and a document image is projected through a lens mounted on a copying optical path onto a uniformly charged photoconductive member for forming an electrostatic latent image thereon. This latent image is developed by applying thereto a powder toner charged to have opposite characteristics to the photoconductive member. The resulting toner image is thereafter transferred and fixed to copying paper, OHP paper, a postcard or the like. In recent years, vigorous developments have been made in the art of producing color copies with a color separation filter mounted on the optical path and a developing device having a plurality of developers including color toners corresponding to the color separation filter. The color separation filter plays a technically important part in such a color copying system, and is required to have high performance.
Generally, the color separation filter comprises an absorption filter which absorbs pigments, or an interference filter which interferes with light. The latter is often preferred in designing the color separation filter because of the greater freedom of designing and the higher efficiency of color separation.
In designing the interference filter, the following three conditions are first determined:
(1) the wavelength of the reflective range,
(2) the width of the reflective range, and
(3) the residual transmittance of the reflective range.
Assume here that the reflective range has a center wavelength T.lambda.o, a width .DELTA..lambda.o and a residual transmittance T.lambda.o. The reflective range centering on wavelength .lambda.o is generated where, as shown in FIG. 3, dielectric elements having a high refractive index nH and those having a low refractive index nL are stacked each in an optical thickness .lambda.o/4 on a substrate G. At this time, the width .DELTA..lambda.o of the reflective range is determined by a ratio between the high refractive index nH and low refractive index nL. Conversely speaking, the width .DELTA..lambda.o of the reflective range is controllable by the ratio between the high refractive index nH and low refractive index nL.
Further, the residual transmittance T.lambda.o of the reflective range is controllable by the number of stacked layers. That is, the residual transmittance T.lambda.o decreases with an increase in the number of layers.
This is illustrated in FIGS. 4 through 7.
The number of layers increases progressively from FIG. 4 to FIG. 7. In this state, the residual transmittance T.lambda.o of the reflective range decreases progressively from FIG. 4 to FIG. 7, with progressively larger ripples occurring at opposite sides of the reflective range. These ripples reduce the transmittance, to the detriment of the color separation filter performance.
FIG. 8 shows a conventional layer arrangement for reducing these ripples. This multi-layer interference filter includes, counted from the substrate G, a first layer S1 of a high-refraction dielectric element, a second layer S2 of a low-refraction dielectric element, a third layer S3 of the high-refraction dielectric element, and so on up to an Nth layer SN of the low refraction dielectric element. The first layer S1 and the last layer SN have an optical thickness .lambda.o/8, respectively, while each of the other layers has an optical thickness .lambda.o/4. This construction is effective for diminishing the ripples at the opposite ends of the reflective range as shown in FIG. 9.
However, the ripples do remain even with this construction, to such a degree to impair highly efficient color separation.